To open with a nerd joke: religion is like Unix in that those who do not understand it are compelled to reinvent it, badly. Watching Sam Harris at a packed Kensington Town Hall last night, it was obvious that he fits squarely into the American tradition of religious leaders who preach liberation from religion into something they call science. He is Mary Baker Eddy for the 21st century.

He was jet-lagged, which may account for some of the incoherence of his position, but he's a very practised performer, and has presumably given this speech hundreds of times before.

What he wants to do is to establish that moral facts exist, and that the division between fact and value is not absolute. This is hardly earth-shaking and certainly not original. Nobody was arguing against it, either on the podium or on the floor: when a show of hands was taken at the beginning of the evening, perhaps a dozen out of at least 1,000 hands went up. The difficulty, of course, comes in establishing what moral facts actually are. This Harris assumes is something to be solved by utilitarian calculation. Understandably he skips over any effort to explain or justify this assumption by argument. Instead he uses a myth.

Consider, he says, "the worst possible misery for everyone". This is a factual state which surely involves a moral obligation to diminish it. So everything which moves away from that, in the long term, is objectively good. And everything which tends to move the world closer to that state is objectively bad.

The obvious retort to this is that our judgements about the way things are tending must involve an element of faith which is something that in other contexts Harris has hoped to escape. But there is a deeper and perhaps less obvious snag.

The argument relies on the assumption that "The worst possible misery for everyone" is an account of the facts: that it is possible to say truly that any state of the universe either is or isn't the worst possible misery for everyone. And that is of course impossible. We could not remotely judge whether something was the worst possible state, any more than we can decide by reason whether everything is for the best in this, the best of all possible worlds. In neither case do we have the materials to make a judgement. We don't have an alternative universe to compare with. And of course to say that the world could be better in the future tells us nothing whatsoever about whether an alternate universe might exist that is better right now.

But even supposing we could identify the state of the greatest possible misery, and we can't, it wouldn't help us to make moral judgements at all. This is because the greatest possible misery is also the greatest possible pain, and the urge to escape pain is entirely pre-moral. It is found in creatures that have no social life and thus no possibility of morality. A world of maximal pain is also a world without utilitarians.

So this is a world view built on a myth in the rather narrow sense. We are invited to deduce moral and factual consequences from a state of the world which may never have existed. Christians, as Giles Fraser pointed out, have had a lot of experience at handling this kind of argument; atheists have rather less, and tend to deal with it by treating their own myths as literal truth.